Conventionally, polyurethane resins have been used in various fields such as the coating industry, adhesives industry, leather-associated industry and the like, as resins which can be used in the boundary field between the rubber field and the plastics field, due to the excellent characteristics such as the mechanical property, wear resistance, chemical resistance, adhesiveness and the like of polyurethane resins.
Although mainstream polyurethane resins are an organic solvent-soluble type polyurethane resins, water-borne polyurethane resins which have water dispersing ability and can reduce a load on the environment and the human body caused by the usage of an organic solvent have been noted in recent years. In order to respond to the increasing social needs such as environmental protection, resource saving and safety, the use of an organic solvent-soluble type polyurethane resin has shifted rapidly to the use of a water-dispersion type polyurethane resin.
As a technology for dispersing a polyurethane resin, a method wherein a mechanical forced emulsion-distribution of a polyurethane resin into water is carried out, a method wherein an ionic group is introduced as a cation or anion into a polyurethane resin and is distributed in water, and the like are known. In terms of specific properties, the performance of the water-borne polyurethane resin is improved to a similar level of an organic solvent-soluble type polyurethane resin due to recent technical progress, and therefore, such a water-borne polyurethane resin has been practically used for various uses.
Among the water-borne polyurethane resins, a self-water-dispersible polyurethane resin to which ionic groups as a cation or anion are introduced has an advantage from the viewpoint of characteristics such that the polyurethane resin can be dispersed without adding strong shearing force, and dispersion stability of the resin in water is comparatively excellent. Particularly, a cationic polyurethane resin has been prevalent in various uses as a highly functional resin which has comparatively excellent corrosive resistance, adhesiveness to a substrate, waterproof characteristics, and the like.
For example, accompanied with market developments corresponding to engineering plastics such as FRP and FRTP, it is required to achieve high performance of a glass fiber which is used as a reinforcer of an engineering plastic. A water-borne polyurethane resin has been used for many years as a glass fiber sizing agent. In recent years, for the purpose of further increasing the sizing property of the glass fiber, the need for a cationic polyurethane resin excellent in adhesiveness with respect to glass fiber has increased.
Various trials have been performed in order to satisfy the aforementioned needs.
For example, a cationic polyurethane aqueous dispersion solution has been proposed which is obtained by using a compound having at least one epoxy group and at least one hydroxyl group in one molecule, polyol, diisocyanate, a chain-extending agent such as N-methyl diethanolamine, N-ethyl diethanolamine and a quaternizing agent. (For example, please refer to patent document 1.) The aforementioned document discloses that the cationic polyurethane aqueous dispersion solution is excellent in ordinary adhesion hardness, waterproof adhesion strength and flexibility, and when it is used as a sizing agent for a glass fiber, the dispersion can provide particularly excellent sizing property and reinforcing effect.
However, the aforementioned dispersion solution has a problem regarding storage stability such that large viscosity change of the dispersion solution arises with the passage of time, due to cohesion of polyurethane resin particles dispersed in water caused while the dispersion solution is preserved. Moreover, the dispersion solution has a practical problem in that the dispersion solution cannot satisfy the market requirement level of adhesiveness to an inorganic substrate such as a glass substrate.
Furthermore, a method for manufacturing a water-dispersible polyurethane is disclosed in which a prepolymer comprising an isocyanate group at the terminal end is prepared by reacting an excess amount of isocyanate with polyol and a cationic hydrophilic-generating agent, wherein the agent is obtained by changing a tertiary amino group of a tertiary amine such as N,N-dimethyl ethanolamine to a quaternary amino group by adding alkylene oxide under the presence of a strong acid such as methanesulfonic acid, and then said obtained prepolymer is dispersed in water to extend a chain formed from the prepolymer. (For example, please refer to patent document 2.)
However, the cationic polyurethane aqueous dispersion obtained as described above also has a problem regarding storage stability, since a large viscosity change arises with the passage of time due to the cohesion of polyurethane resin particles dispersed in water caused while the solution is preserved. Moreover, similar to the aforementioned solution, the aqueous dispersion has a practical problem in that it cannot satisfy the market requirement level of adhesiveness to an inorganic substrate such as a glass substrate.
On the other hand, in the ink-jet print industries which have been rapidly developed in recent years, an image quality formed by an ink-Jet printer has remarkably improved and it is possible to obtain an image which is no less excellent than silver halide photos. In order to achieve a high fine image, it is necessary to use an ink-jet recording medium excellent in ink absorbing property, high color density, bleeding preventing property, gloss and the like.
Moreover, the ink-jet printer is used not only as a household printer but also for business uses such as signboards and advertisements. So-called wide-format ink-jet printers, which can conduct a large-size printing, are used for said business uses. Such types of printer eject excessively large amounts of ink as compared with household ink-jet printers. Therefore, an ink-jet recording medium usable for a wide-format inkjet printer is required to achieve furthermore excellent high ink absorbing property as compared with the household printer. Furthermore, in many cases, the printed medium tends to be exposed to the outdoor for a long period of time. Accordingly, the medium is required to be excellent in waterproof characteristics, resistances to light and gases such as ozone, and also excellent in image preservation property.
As an improving means from the ink side, although dye has been conventionally used as a coloring material of an ink-jet ink, the employment of a method which uses a pigment ink instead of dye ink has been utilized in order to improve the image preservation property. Although the pigment ink was only used for the wide-format ink-jet printer for business use at first, the pigment ink has also been employed for some models of household ink-jet printer in recent years.
By the way, a water based ink is used for the ink-jet printer in general. Usually, an ink-jet recording medium includes a substrate such as paper and a plastic film and an ink-jet receiving layer provided thereon, wherein the layer is formed from an ink-jet receiving agent which is a water-soluble resin such as polyvinyl alcohol, polyvinyl pyrrolidone and the like and any of various additives, in order to prevent bleeding caused by a water based ink or improve ink absorbing property.
However, the inkjet receiving layer which is popularly used at present is a layer developed for a dye ink which has been conventionally used. Therefore, when such an ink-jet receiving layer is used for a pigment ink, the employment of which has increased in recent years, problems are caused such that bleeding arises due to insufficient adsorption of the pigment ink into the ink-jet receiving layer, a homogenized image cannot obtained due to the occurrences of cracks which are formed in an image portion where the printed amounts of ink is large, and the like. In this way, such a receiving layer cannot be adapted to printing of a pigment ink.
Moreover, a water based ink is used for ink-jet printing in general as described above, and therefore, there is a problem that a printed image printed by the ink-jet printer has poor waterproof characteristics. Accordingly, various trials have been conducted in order to improve the poor waterproof characteristics.
The most popular method to improve the waterproof characteristics is a method wherein an ink-jet receiving agent is used which includes an aqueous cationic resin such as a poly(diallyldimethylammonium chloride) in addition to the aforementioned water soluble resin such as polyvinyl alcohol and polyvinyl pyrrolidone. In the method, waterproof characteristics can be improved by fixing of coloring material (dye and/or pigment) of a water based ink due to the electrostatic bonding between an anionic group included in the molecule of the coloring material (dye and pigment) in the ink and a cationic group in the molecule of the water-soluble cationic resin. Due to the method, the waterproof characteristics can be improved to some extent. However, since the water-soluble cationic resin itself tends to be easily dissolved in water, the effect of the method for improving waterproof characteristics was insufficient. Moreover, such an ink-jet receiving layer has poor printing property and cannot achieve a level usable for an ink-jet receiving layer for a pigment ink.
On the other hand, various ink-jet recording media which comprise an ink-jet receiving layer containing large amounts of porous inorganic fine particles, so-called micro porous type or a void type recording media, have been proposed as an ink-jet recording medium which can adsorb an ink quickly and is excellent in quick-drying capability. For example, a receiving sheet which includes an inkjet receiving layer comprising a pseudo boehmite type alumina (for example, please refer to patent document 3), and an ink-jet receiving sheet, which is excellent in gloss and waterproof characteristics and includes an ink-jet receiving layer comprising a silica, aluminum compound produced by a fumed method and the like (for example, please refer to patent documents 4) have been proposed.
These ink-jet receiving layers have innumerable fine voids, and the fine voids enable the layer to be printed due to the adsorption of a solvent of an ink into the voids. However, there is a limit to the void quantities to be provided. Therefore, when printing is conducted with a printer, which ejects large amounts of ink, such as a wide-format ink-jet printer in which pigment inks are mainly-used, a problem arises such that good quality printed materials cannot be obtained since fine voids of a receiving layer cannot adsorb all of the solvent of the ink and thus non-absorbed ink overflows therefrom.
Moreover, since such an ink-jet receiving layer contains large amounts of inorganic fine particles, fine cracks tend to be formed at the coated surface thereof, when an ink-jet receiving agent is coated on a substrate and dried. Accordingly, it is necessary for an ink-jet receiving layer to be dried at low temperature, and/or to conduct coating plural times in order to form an ink-jet receiving layer wherein each coating does not provide a thickness sufficient to allow cracks. Therefore, there was a problem in that manufacturing efficiency was remarkably poor.
As described above, there is great demand for the development of an ink-jet receiving agent which can achieve good manufacturing efficiency of an ink-jet recording medium, and can provide an ink-jet receiving layer which is excellent in printing ability and absorbing property with respect to a pigment ink, and which is excellent in waterproof characteristics of a printed image formed on the medium.    Patent document 1: Japanese Unexamined Patent Application, First Publication, No. 58-219213    Patent-document 2: Japanese Unexamined Patent Application, First Publication, No. 5-320331    Patent-document 3: Japanese Unexamined Patent Application, First Publication, No. 2-276670    Patent-document 4: Japanese Unexamined Patent Application, First Publication, No. 2000-309157